I have a 2001 Catera and recently had the check engine light come up giving the codes 0171 0174 which relate to lean banks one and two and a central vacuum leak. Now, I replaced the plenum gasket/o-ring and the other 2 smaller o-rings in the vacuum lines, reset the codes and the next day the Check Engine Light popped back on. I've sprayed carb cleaner all around the vacuum lines, and plenum an no change in the engine revving up or anything of the sort. I changed the gas cap too. Reset the computer again and the light comes back on in a matter of a few miles.

So, I'm going to try some Super Unleaded in it (I always put mid grade in) next. Then it's going to be changing the fuel filter, then the MAP sensor.

I know a few posts relate to these two error codes and users have solved the problem with the same methods